Mixtures of liquids whose vapor and liquid phases in equilibrium have identical compositions are known as azeotropes. The boiling of such mixtures occurs at a constant temperature and therefore the components do not separate. Azeotropes find particular application as refrigerants because the composition of the liquid in the evaporator is always the same as the vapor boiling out of the evaporator and passing through the compressor.
There are two types of azeotropic mixtures, one in which the pressure of the mixture at a given temperature is less than the pressure at that temperature of either of its components. The other more common azeotrope is one in which the evaporating or condensing pressure at a given temperature is greater for the mixture than for either component alone or, equivalently, at a given pressure the temperature of the azeotrope is lower than that of either pure component. The latter is known as a maximum pressure (or minimum temperature) azeotrope and an example is the R-12/R-40 refrigerant described in U.S. Pat. No. 2,547,887 for use in a centrifugal compressor operated at relatively slow speed.
It is a principal purpose of this invention to provide a new refrigerant mixture having a higher saturated vapor density at a given temperature than that of either of its components alone, thus enabling a centrifugal compressor, or any other compressor which operates on the principle of converting kinetic energy of the vapor to potential energy, to operate over a greater lift without surging or other flow instabilities. A principal object is to provide a substantially azeotropic refrigerant which will permit an existing centrifugal compressor system designed only to chill water to operate at substantially lower temperatures and be capable of freezing ice, all without structural or electrical change to the compressor. Another advantage of the greater lift in chilling water is that operation can be at a higher condensing temperature and therefore an air-cooled dry condenser can be used instead of a wet cooling tower or evaporative sprayed condenser.
A further application of the mixture of the invention is to permit the use of standard centrifugal compressors in hotter or more humid climates than those in which they can now operate, both for chilling water and for making ice. The new refrigerant also can be used to cure surging in compressors in field operation which occurs when compressors cannot meet required lift conditions. Yet another feature of the invention is that it gives a wider latitude of operation under partial-load conditions when less refrigerant load is required but the same lift between evaporator and condenser pressures is still needed.